Ducted gas turbine engines usually comprise a core engine which drives a propulsive fan assembly. The fan assembly comprises a number of radially extending aerofoil blades mounted on a common hub and enclosed within a generally cylindrical casing assembly. The fan assembly and casing assembly are encircled by a generally annular nacelle assembly which forms the air intake of the engine and is aerodynamically shaped. The nacelle may extend both forward and rearward, relative to the direction of airflow, of the fan assembly.
There is a remote possibility with such engines that part or all of one or more of the fan blades could become detached from the remainder of the fan assembly. The occurrence of a part or all of one or more of the fan blades becoming detached from the fan assembly and impacting the casing assembly is hereinafter termed a FBO (fan blade off) event. The casing assembly surrounding the fan assembly is specifically designed to contain the detached blade or blade portion. However, it is important that the nacelle is not damaged during the FBO event as the casing assembly is subject to distortion. It is also important to remove the possibility of further damage to the nacelle, after the FBO event, resulting from vibrations during run down and subsequent windmilling due to the fan assembly being out of balance. Run down being hereinafter defined as the deceleration of axial rotational speed of the engine from the rotational speed at which a fan blade or part of a fan blade has been released and caused safety systems to shut down the engine. Windmilling being hereinafter defined as the axial rotation of the fan assembly arising from air ingressing the engine due to the forward speed of the aircraft after engine shut down.
Typically the nacelle assembly may be attached to a component of the engine and/or an engine support pylon assembly with the necessary access to the engine and engine core mounted accessories usually made by either opening fan cowl doors located in the body of the nacelle as described in WO93/02920 or by the nacelle assembly comprising two part-circular portions acting in a clam-shell like manner as described in U.S. Pat. No. 5,205,513. Furthermore, the nacelle assembly is commonly attached to the fan casing as described in U.S. Pat. No. 4,044,973, with such attachments being required to be particularly robust to maintain attachment after a FBO event. The nacelle designs, in particular the attachment means to the engine and/or pylon, of the prior art herein cited lend themselves to complex and heavy, thus expensive, assemblies. The nacelle assemblies also appear to be prone to damage during a FBO event and subsequent vibrational damage caused by windmilling of the out of balance fan assembly during fly home.